Ductile Fracture Simulation Of API X80 Pipeline

Offshore oil and gas pipeline is an important structure in the process of safe exploitation,production and transportation of oil and gas.With the development of oil and gas gradually going to the deep sea,the service environment of oil and gas pipelines becomes more stringent,and the consequences are usually very serious once the offshore oil and gas pipelines are damaged.Therefore,it is very important to ensure the strength and safety of oil and gas pipelines in the design stage.Because API X series steel pipe of oil and gas belongs to high toughness material,plastic deformation of pipeline is allowed in the design process of oil and gas pipeline.In order to accurately calculate the ultimate load of oil and gas pipelines under large deformation conditions,the study of ductile fracture behavior of oil and gas pipelines is an indispensable part.In this paper,the ductile fracture behavior of API X80 steel pipe is studied by damage mechanics model.The main research contents are as follows:(1)Firstly,the research status of ductile fracture of steel pipes at home and abroad is summarized,and two kinds of fracture theories,fracture mechanics theory and damage mechanics theory,are introduced.Because the theory of fracture mechanics does not explicitly consider the influence of stress state,it has some limitations.In this paper,only the damage theory is discussed.This paper mainly introduces the micro-based GTN model and the two phenomena-based uncoupled fracture models:MMC model and ERT model.(2)The plasticity of X80 pipeline steel was calibrated based on uniaxial tensile test.The parameters of GTN,MMC and ERT models were calibrated based on the hybird numerical-experimental calibration procedure.Five different kinds of notched specimens were tested to obtain the fracture properties of X80 pipeline steel under different stress states,and the corresponding finite element models were established.According to the comparison between the finite element model and the test results under different parameters of GTN model,nine parameters of GTN model are tried out.The parameters of MMC and ERT model are obtained by the simulation of non-damage finite element model,and the corresponding stress state history curve is extracted.The problem of parameter determination is transformed into an unconstrained optimization problem with three parameters,which is optimized by quasi-Newton method.(3)Using ABAQUS subroutine VUMAT and backward Euler algorithm,MMC and ERT fracture models are added to the finite element model.According to the calibrated model parameters and the element deletion technique,the ductile fracture of five kinds of notch tests was simulated,and the feasibility of the model was preliminarily verified.Compact tension(CT)tests commonly used in fracture mechanics methods are used to analyze the overall corresponding conditions and crack propagation of the specimens.The optimum mesh sizes of GTN,MMC and ERT models are determined by grid sensitivity analysis,and the relationship between them and traditional fracture mechanics methods is shown.(4)Based on GTN,MMC and ERT models,the internal pressure resistance of 21 inch X80 steel pipe with initial axial defects is analyzed.The results show that the three models have similar results.The failure process of pipe wall separation can be simulated by damage model combined with element deletion technology.Through the study of ASME and DNV specifications,two main factors affecting blasting pressure are determined:defect length and defect depth.The MMC model is used to analyze the blasting pressure of pipelines with different defects.The simulation results are in good agreement with the specifications.Furthermore,the influence of different axial forces and curvatures on the internal pressure resistance of pipelines is studied by using MMC model.